1. Field of the Invention
The present invention relates to methods of making low-stress, large-volume crystals, which have reduced birefringence over their volume and a uniform index of refraction, by means of a specially directed tempering. The invention also relates to the crystals made by the method, to optical elements made from the crystals and generally to uses of the crystals made by the method.
2. Related Art
There is an ever-increasing need for crystals, especially single crystals, for lenses and optical systems, also for wavelengths outside of the visible range. For example, lasers generating laser beams at wavelengths in the far UV range (DUV), especially at wavelengths below 250 nm, especially at 248 nm, 193 nm and/or 157 nm, are used in methods of making computer chips by means of microlithography. Large-sized single crystals are used as optical elements and/or lenses in illuminating and imaging optics at 193 nm and preferably at 157 nm. CaF2 has been used as a preferred material for this purpose. Optics for manufacture of integrated circuits, e.g. computer chips, must have the smallest possible structural defects, i.e. sharp imaging, in order to obtain a perfect product. In order to attain the required image quality, very high specifications are established for the optical base material, i.e. the crystal. Thus the index of refraction n must be as uniform as possible, i.e. its variation in the lens blank should not be more than 1*10−6 and the stress birefringence should be definitely under 1 nm/cm.
The stress optical tensor gives the connection between the mechanical variables (such as stress) and the optical effects caused by them (such as the stress birefringence SDB) in crystals (direction-dependent). That means that stresses of equal magnitude in single crystal material can lead to clearly different stress birefringence and non-uniformities in the index of refraction depending on the crystallographic orientation and/or the observation direction. For these reasons up to now components were used, in which the minimal stress birefringence is obtained in the application or use direction. For calcium fluoride crystals that direction is the (111) direction. Thus currently materials for lens blanks are used exclusively in the (111) orientation (and/or near the (111) direction for cubes).
A number of methods for making large-volume single crystals are known in the art. For example DE-A 100 10 484 describes an apparatus for growing large-volume single crystals. This reference also describes a method for tempering this sort of crystal to reduce the stress birefringence. In this method usually the method starts with crystals with an average stress birefringence (RMS value) of about 5 to 20 nm/cm prior to tempering. The described method may reduce the stress birefringence in the crystals to 1 nm/cm and reduces the refractive index variations so that the refractive index does not vary by more than Δn=5*10−6 within the crystal. After being grown the crystal is placed in a tempering oven and heated for at least two hours above 1150° C. in the presence of a CaF2 powder reducing the evaporation from the crystal. Also a tempering process is described in DD-PS 213,514, in which a CaF2 crystal is heated in a PbF2 containing atmosphere at a temperature of 1200° C. In this tempering process stress birefringence of 10 to 25 nm/cm (RMS value), which is present in the crystal, is reduced to only 1 nm/cm by heating for 2 to 3 hours at 1200° C.
In JP-A 2001-335 398 a tempering method for reducing stress birefringence and including uniformity of refractive index is described, in which the crystal is first heated in a range from 1020 to 1150° C. for a predetermined time and is cooled with a cooling rate of 1° C. per hour, then to a temperature under 700° C.
In JP-A 10-251 096 a procedure is described in which the crystal is first cut to the dimensions of the final product and subsequently a tempering process is performed.
Crystals with the smallest possible dimensions must be tempered based on theoretical considerations regarding the relationships between the dimensions of the crystal body, such as diameter and thickness, and the stresses producing the stress birefringence occurring in the material. Since the temperature differences ΔT producing stress produced by heating and/or cooling between different locations in the crystal are proportional to the square of the thickness and/or height, h, of the crystal and the square of the diameter, d, of the crystal disk, i.e. ΔT˜d2 and ΔT˜h2, crystals with the smallest possible volume are to be tempered. An increase in the diameter and/or the height or thickness of the crystal body during tempering causes an increase in the stress birefringence, other conditions being equal.
During tempering a nearly complete elimination of the stresses present in the crystal occurs by a plastic deformation of the crystal lattice by activation of glide processes along a glide system predetermined by the respective structure of the crystal lattice and by atomic diffusion mechanisms (cascade processes). Thus it is necessary that the tempering or annealing temperature is as high as possible, in order to obtain a crystal in which the defects are minimized throughout its entire crystalline volume. Generally the higher the temperature, the less the residual stress. However the crystal must be given sufficient time for the desired glide and diffusion processes to occur. During the entire tempering process comprising the heat-up stage, the holding stage when the temperature is held at a maximum value and the cooling stage, an extremely uniform temperature distribution must be guaranteed throughout the entire crystal volume.
The spatial temperature distribution throughout the entire crystal volume results from an overlap of a static temperature gradient (apparatus dependent temperature distribution) with a dynamic temperature gradient, which arises because of the heating up and cooling down of the crystal. The former gradiens dominates the holding time, the latter comes into play during the heat up and cool down of the crystal. Based on those considerations currently the orientation of the crystal in the temperature field is considered irrelevant for the tempering results, since the volume at the maximum temperature (holding temperature) is essentially in a more uniform static temperature field than that in the heat up stage or the cooling down stage.
In order to keep the crystal structure produced by the tempering more or less stress-free, it is cooled in such manner that no temperature gradient, which produces new stresses in the crystal, is produced. It has been shown that stresses in materials first lead to elastic and later to plastic deformation when a critical value is exceeded.